This invention relates to radio receivers having a circuit for displaying a received radio wave frequency.
There are two ways for displaying a received frequency: one is an analog display, and the other is a digital display. The latter is more advantageous than the former, because with the digital display a value displayed can be directly read so that the frequency value reading is carried out accurately and quickly.
In a conventional circuit for digitally displaying a received frequency in a radio receiver performing the superheterodyne reception, the oscillation output of the local oscillator is applied to a counter in which a value corresponding to the intermediate frequency is preset so as to be added to or subtracted from the received-frequency value, and the count result is digitally displayed.
However, with such a digital display circuit, the frequency displayed is not the frequency of the actually received broadcast radio wave, because the displayed frequency is obtained by correcting the local oscillation frequency with the intermediate frequency component. Accordingly, the conventional digital display circuit suffers from a disadvantage that if the radio receiver is not completely tuned to the frequency of the broadcast radio wave, it is not accurately displayed. For instance, when it is intended to receive a broadcast radio wave of 1000 KHz, it can be received if it is within the tuning range of the radio receiver determined by the band width of its tuning circuit. However, it should be noted that the displayed frequency is varied within the aforementioned tuning range in correspondence to the change of the oscillation frequency of the local oscillator. In other words, even if the radio receiver is somewhat detuned, the broadcast radio wave frequency can be received by catching it within the tuning band width thereof. However, since the frequency display is effected in accordance with the local oscillation frequency but not with the received frequency, the frequency displayed thereby is different from the received frequency if the radio receiver is not accurately tuned thereto. Furthermore, if the output of the local oscillator drifts, correct frequency display is not carried out.